Extracting petroleum essentially involves extracting a petroleum/water mixture from the ground. In newly developed oil fields, this mixture consists of the formation water already present in the oil-bearing stratum and the oil which is to be extracted. In older oil fields, extraction of the petroleum/water mixture involves forcing water into the petroleum formation through injection wells which are located at a certain distance from the extraction well. Under the pressure developed in this manner, the water flows in the oil-bearing stratum to the extraction well and, in so doing, carries petroleum with it which may then be taken from the extraction well together with the injection water and any formation water which is present.
This petroleum/water mixture now has to be separated in order to obtain petroleum which is suitable for further processing. The water content of the petroleum/water mixture varies from oil field to oil field and also varies as exploitation of the oil field progresses.
When refining petroleum, oil products containing salt for example must be desalted. The oil product is washed with water for this purpose. Water contaminated with oil is obtained as waste product.
Oil/water mixtures are very harmful to the environment and therefore cannot be discharged untreated into the environment, for example requiring elaborate disposal arrangements. It is therefore in principle desirable to treat oil/water mixtures by separating the oil as completely as possible from the water in order to obtain clarified water which may then for example be put to further use.
Phase separators are generally used for separating the oil/water mixture, i.e. the oil/water mixture is transferred into a tank in which the lighter oil and the heavier water can segregate under the influence of gravity, such that an oil phase forms at the surface and an aqueous phase (for example deposit water in the case of petroleum extraction) in the lower region of the tank.
After such a gravity separation, between 500 and 1000 mg of oil are still present per liter of water. On the one hand, this involves a considerable waste of precious oil and, on the other hand, the water cannot be discharged into the environment due to the considerable contamination with oil but must instead be further treated and purified.
Currently available treatment methods and devices, however, have considerable disadvantages with regard to costs and treatment performance. In the case of the three-phase separators usual in the United Arab Emirates, a residual oil content of 500 ppm oil remains in the wastewater. Disposal of the water is extremely problematic at such a high loading. Sometimes the water is transferred into special wastewater lagoons which involves considerable environmental pollution. Alternatively, the wastewater is sometimes forced under elevated pressure into porous aquifers. While this practice does indeed initially get rid of the water, it makes groundwater deposits useless for future extraction of drinking water.
In German oil fields, the residual oil content is often reduced to around 60 ppm by means of coalescence separators. However, even this loading is still too high for direct discharge of the water, such that here too the water can only be discharged to an effluent treatment plant, so causing considerable additional costs.
A solution which is at first glance obvious in petroleum extraction of using the contaminated deposit water as injection water for the injection wells is in many cases likewise not possible due to the residual oil content of the water. If injected into the oil field, the oil particles can clog the pores in the oil-bearing formation and so considerably reduce output from the oil field and, in extreme cases, may bring extraction to a virtual standstill. Even if it is possible to force the deposit water back into the oil field, there are further related problems. This is because, if the contaminated water is to be safely stored, it would have to be forced precisely into the conductive oil-bearing reservoir which has been developed by the extraction well. Such precise input is, however, almost impossible to ensure reliably. There is therefore always a risk that the introduced deposit water will not remain in the oil field, but will instead be forced into other rock strata where it may cause contamination, in extreme cases in the aquifer too.
Ultrafiltration of deposit water containing oil has repeatedly been investigated. However, such investigations revealed the problem that the membranes used are likewise irreversibly clogged by the oil particles within a short time. While using ceramic membranes has made backflushing feasible, such that the membranes can be used over an extended period, the service period of these special ceramic membranes between flushing operations is very short and the membrane costs are very high. Methods using these membranes can thus only be considered in a few specific cases involving relatively small volumes of deposit water.